Enhanced Photocatalytic Hydrogen Evolution over Cu-Doped ZnIn2S4

16148

J. Phys. Chem. C 2008, 112, 16148–16155

Enhanced Photocatalytic Hydrogen Evolution over Cu-Doped ZnIn2S4 under Visible Light Irradiation Shaohua Shen, Liang Zhao,* Zhaohui Zhou, and Liejin Guo State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong UniVersity, Xi’an, ShaanXi 710049, P. R. China ReceiVed: May 22, 2008; ReVised Manuscript ReceiVed: July 25, 2008

A series of Cu-doped ZnIn2S4 photocatalysts has been synthesized by a facile hydrothermal method, with the copper concentration varying from 0 wt% to 2.0 wt%. The physical and photophysical properties of these Cu-doped ZnIn2S4 photocatalysts were characterized by X-ray diffraction (XRD), photoluminescence spectroscopy (PL), scanning electron microscopy (SEM), and UV-visible diffuse reflectance spectroscopy (UV-vis). The diffuse reflectance and photoluminescence spectra of Cu-doped ZnIn2S4 shifted monotonically to longer wavelengths as the copper concentration increased from 0 wt% to 2.0 wt%, indicating that the optical properties of these photocatalysts greatly depended on the amount of Cu doped. Meanwhile, the layered structure of ZnIn2S4 would be destructed gradually by Cu doping. The photoactivity of ZnIn2S4 was enhanced when Cu2+ was doped into the crystal structure. The highest photocatalytic activity was observed on Cu (0.5 wt%)–doped ZnIn2S4, with the rate of hydrogen evolution to be 151.5 µmol/h under visible light irradiation (λ > 430 nm). On the basis of the calculated energy band positions and optical properties, the effect of copper as a dopant on the photocatalytic activity of Cu-ZnIn2S4 was studied. 1. Introduction Photocatalytic water splitting into H2 and O2 using semiconductor photocatalysts has been regarded as an attractive solution to resolve the global energy and environmental problems. Many effective photocatalysts have been developed; however, most of them only work in the ultraviolet region because of their wide band gap.1-4 Thus, developing highly active photocatalysts with visible light response to use solar energy is important, as the ultraviolet light occupies only 2-3% of solar energy. Over the past few years, considerable effort has been made to narrow the band gap of catalysts to improve visible light response. Recent progress includes single-phase oxide PbBi2Nb2O9 for isopropyl alcohol degradation and water decomposition,5 GaN-ZnO solid solution,6 and titanium disilicide7 for pure water decomposition to H2 and O2. One effective way to increase the absorption of photocatalyst is to form donor levels in the forbidden band by introducing foreign elements into active photocatalysts with a wide band gap. TiO2 codoped with Ni and Ta,8 ZnS doped with Cu or Ni,9,10 and ZnS codoped with Pb and halogens11 have been reported as active photocatalysts under visible-light irradiation. In particular, the InTaO4 doped with Ni was found to be an active photocatalyst for overall water splitting in the visible light region.12 On the other hand, chalcogenides such as CdS have been studied extensively, since they have ideal edge positions of the valence and conduction bands for hydrogen production from aqueous electrolyte solutions containing a sacrificial agent (Na2S or/and Na2SO3) under visible light irradiation.13,14 Unfortunately, they also have a fatal disadvantage of photocorrosion, i.e., photogenerated holes in the valence band tend to react with chalcogenides themselves.15 Although incorporation of chalcogenides into interlayers16 or mesoporous materials17,18 was efficient for stabilizing the chalcogenides and producing hy* To whom correspondence should be addressed. Phone: +86-2982668287. Fax: +86-29-82669033. E-mail address: [email protected].

drogen from water, the photocatalytic efficiency is still low. Recently, some multicomponent metal sulfides have been reported to be more stable and show higher photocatalytic activity under visible light.19-21 Moreover, the CdZnS doped with Cu showed greatly improved stability and photocatalytic activity even without Pt as a cocatalyst.22 Ternary sulfide ZnIn2S4, as the only member of the AB2X4 family semiconductor with a layered structure, has attracted farranging interests because of its potential applications in different fields such as charge storage,23 thermoelectricity,24 and photoconduction.25 In 2003, Lei et al.26 synthesized ZnIn2S4 by a simple hydrothermal method and first treated ZnIn2S4 as an efficient visible-light-driven photocatalyst for hydrogen production. In 2006, Gou et al.27 hydrothermally prepared the ZnIn2S4 solid or hollow microspheres in the presence of a surfactant such as cetyltrimethylammonium bromide (CTAB) or poly(ethylene glycol) (PEG). In our previous research,28,29 the photocatalytic activity of ZnIn2S4 could be further enhanced when hydrothermally prepared in the presence of CTAB. Thus, ZnIn2S4 turned to be a good candidate for hydrogen production from water under visible light. In the present study, a series of ZnIn2S4 doped with different amounts of Cu was synthesized by a CTAB-assisted hydrothermal method. The effects of Cu doping on the morphology, optical properties, and photocatalytic activities of ZnIn2S4 were discussed in detail. 2. Experimental Section 2.1. Synthesis of Cu-Doped ZnIn2S4. All chemicals are of analytical grade and used as received without further purification. ZnIn2S4 products were prepared by a hydrothermal synthetic method.27 In a typical procedure, 0.735 g of ZnSO4 · 7H2O, 1.615 g of In(NO3)3 · 4H2O, 0.65 g of cetyltrimethylammonium bromide (CTAB), and a double excess of thioacetamide (TAA) were dissolved in 50 mL of distilled water. The mixed solution was then transferred into a 70-mL Teflon-lined autoclave. The autoclave was sealed and kept at 160 °C for 12 h

10.1021/jp804525q CCC: $40.75  2008 American Chemical Society Published on Web 09/20/2008

Enhanced Photocatalytic Hydrogen Evolution

J. Phys. Chem. C, Vol. 112, No. 41, 2008 16149

Figure 1. X-ray diffraction patterns of Cu(X)-ZnIn2S4; the values of X were (a) 0.0 wt%, (b) 0.1 wt%, (c) 0.3 wt%, (d) 0.5 wt%, (e) 0.7 wt%; (f) 0.9 wt%, (g) 1.2 wt%, (h) 1.6 wt%, (i) 2.0 wt%.

and then cooled to room temperature naturally. A yellow precipitate was obtained, which was then filtered and washed with absolute ethanol and distilled water for several times. After being dried in a vacuum at 80 °C, ZnIn2S4 was obtained. For the synthesis of Cu-doped ZnIn2S4, a desired amount of 0.1 M Cu(NO3)2 solution was added into the mixed solution before transferring into the autoclave. ZnIn2S4 doped with different amounts of Cu is denoted as Cu(X)-ZnIn2S4; the value of X, from 0 wt% to 2.0 wt%, is used to describe the weight content of Cu. 2.2. Characterization. The X-ray diffraction (XRD) patterns were obtained from a PANalytical X’pert MPD Pro diffractometer using Ni-filtered Cu KR irradiation (Wavelength 1.5406 Å). The diffuse reflection of the samples was determined by a Hitachi U-4100 UV-vis-near-IR spectrophotometer. The analysis of photoluminescence spectra (PL) was carried out at room temperature using a PTI QM-4 fluorescence spectrophotometer. The sample morphology was observed by a JEOL JSM6700FE scanning electron microscope. 2.3. Evaluation of Photocatalytic Activity. Photocatalytic hydrogen evolution was performed in a gas-closed circulation system with a top window Pyrex cell. A 300 W Xe lamp was used as the light source, and the UV part of the light was removed by a cutoff filter (λ > 430 nm). Hydrogen evolved was analyzed by an online thermal conductivity detector (TCD) gas chromatograph (NaX zeolite column, nitrogen as a carrier gas) every 10 min. In all experiments, the photocatalyst powder (0.2 g) was dispersed by a magnetic stirrer in 150 mL of 0.25 M Na2SO3/0.35 M Na2S aqueous solution. Here, Na2SO3/Na2S mixed sacrificial agent was used to scavenge photogenerated holes. Nitrogen was purged through the cell before reaction to remove oxygen. Pt (1 wt%) as a cocatalyst for the promotion of hydrogen evolution was photodeposited in situ on the photocatalysts from the precursor of H2PtCl6 · 6H2O. The temperature for all the photocatalytic reactions was kept at 25 ( 0.5 °C. The blank experiments showed no appreciable H2 evolution in the absence of irradiation or photocatalyst. Apparent quantum yield defined by the eq 1 was measured using a 420 nm band-pass filter and an irradiatometer.

the number of reacted electrons × 100 the number of incident photons (1) the number of evolved H2 molecules × 2 ) × 100 the number of incident photons

A.Q.Y.(%) )

2.4. Band Structure Calculation. The plane-wave-based density functional theory (DFT) calculation was carried out for ZnIn2S4 with hexagonal structure by employing the CASTEP

program to obtain further information about the energy structure of the Cu-ZnIn2S4 photocatalysts.30 The core electrons were treated using ultrasoft pseudopotentials,31 and the valence electronic configurations for Zn, In, and S are 3d104s2, 4d105s25p1, and 3s23p4, respectively. The kinetic energy cutoff was taken to be 400 eV for ZnIn2S4. The exchange and correlation interactions were modeled using the generalized gradient approximation (GGA). The calculation was carried out using the primitive unit cell of [ZnIn2S4]2, which had 31 occupied orbitals. The atomic coordinates of ZnIn2S4 were obtained from Gou et al.27 3. Results and Discussion 3.1. Crystal Structure. Figure 1A shows X-ray diffraction patterns of Cu(X)-ZnIn2S4 photocatalysts (X ) 0 wt% to 2.0 wt%). Without Cu doping, the diffraction peaks of the obtained ZnIn2S4 sample can be readily indexed as a pure phase hexagonal ZnIn2S4 (ICSD-JCPDS card No. 01-072-0773, a ) 3.85 Å, c ) 24.68 Å), and no other impurities such as ZnS, In2S3, oxides, or organic compounds related to reactants were detected. When ZnIn2S4 was doped with Cu, the XRD patterns of the obtained Cu(X)-ZnIn2S4 were almost the same as that of the hexagonal ZnIn2S4, except that the diffraction peaks were slightly shifted to higher angles with the amount of Cu doped increasing, as shown in Figure 1B. This observation suggested that Cu2+ was homogeneously incorporated into the lattice of ZnIn2S4, because the ionic radius of Cu2+ (0.72 Å) is smaller than the radius of Zn2+ (0.74 Å) and In3+ (0.81 Å).32 Furthermore, Cu2+ may occupy the Zn2+ site, since charge compensation was very easy in this case, as deduced by Shionoya et al.33 An XRD pattern of CuS around 2θ ) 30° could be easily observed while the amount of Cu doped was close to 2.0 wt%. This was due to the limitation of doping amount in the ZnIn2S4 lattice.11 3.2. Morphology. Figure 2 shows SEM images for Cu(X)ZnIn2S4 with X ) 0 wt% to 2.0 wt%. During the hydrothermal process, addition of Cu ion directly affects the morphology of Cu-ZnIn2S4. When none of the Cu was doped, ZnIn2S4 exhibited a lot of separate microspheres with diameters of 1-2 µm, as shown in Figure 2a. Further observation showed that all the microspheres were composed of numerous petals/sheets. This growth tendency of lamellar structures might be related to the layered feature of hexagonal ZnIn2S4.27 When X ) 0.1 wt% to 0.5 wt %, ZnIn2S4 products still appeared in the shape of microspheres; however, increasing the amount of Cu doped led to gradual decrease in the diameter of Cu(X)-ZnIn2S4 microspheres. The diameter of microsphere was about 0.5-1 µm for

16150 J. Phys. Chem. C, Vol. 112, No. 41, 2008

Figure 2. Part 1 of 2.

Shen et al.



Figure 2. Part 2 of 2. Scanning electron microscope images of Cu(X)-ZnIn2S4; the values of X were (a) 0.0 wt%, (b) 0.1 wt%, (c) 0.3 wt%, (d) 0.5 wt%, (e) 0.7 wt%; (f) 0.9 wt%, (g) 1.2 wt%, (h) 1.6 wt%, (i) 2.0 wt%.

Cu(0.5 wt%)-ZnIn2S4. As X increased further, the shape of microspheres for Cu(X)-ZnIn2S4 was destructed partially and even throughly, as shown in Figure 2e-g, as well as the petals/ sheets presented to be more irregular. When X ) 1.6 wt% to 2.0 wt %, instead of microspheres and petals/sheets, Cu(X)ZnIn2S4 turned out to be a bulky conglomeration with a rough surface, and petals/sheets could be hardly observed. The morphology of the Cu(X)-ZnIn2S4 samples obtained in the same synthetic condition varies regularly with the increasing amount of Cu doped. That is to say, the layered structure of hexagonal ZnIn2S4, as well as the shape of microspheres, could be destructed gradually by the increasing Cu dopant. 3.3. Optical Properties. Figure 3 shows the UV-vis diffuse reflectance spectra of Cu(X)-ZnIn2S4 with X ) 0 wt% to 2.0 wt%. The onset of the absorption edge of ZnIn2S4 was at about 495 nm, corresponding to the band gap of 2.51 eV. Moreover, ZnIn2S4 had an intense absorption band with a steep edge in the visible-light region. This shape indicated that the UV-vis absorption was due to the band gap transition but not due to the transition from impurity levels to the conduction band of ZnIn2S4. As X increased, a new absorption shoulder around 500 nm was observed, and the absorption edge of Cu(X)-ZnIn2S4 shifted monotonously from 575 to 650 nm with an increase in

Figure 3. Diffuse reflectance spectra of Cu(X)-ZnIn2S4; the values of X were (a) 0.0 wt%, (b) 0.1 wt%, (c) 0.3 wt%, (d) 0.5 wt%, (e) 0.7 wt%; (f) 0.9 wt%, (g) 1.2 wt%, (h) 1.6 wt%, (i) 2.0 wt%.

X from 0.1 wt% to 1.2 wt%, corresponding to the narrowing of the band gap from 2.16 to 1.91 eV. The absorption shoulder with a long tail on the low energy side, which was characteristic of doped photocatalysts34,35 and indicated that intraband gap states were formed by the dopants in the forbidden band, could


Shen et al.

Figure 4. Photoluminescence spectra of Cu(X)-ZnIn2S4; the values of X were (a) 0.0 wt%, (b) 0.1 wt%, (c) 0.3 wt%, (d) 0.5 wt%, (e) 0.7 wt%; (f) 0.9 wt%, (g) 1.2 wt%, (h) 1.6 wt%, (i) 2.0 wt%, excited at 330 nm.

TABLE 1: Band Gap, Absorption Edge, PL Emission and Emission Intensity of Cu(X)-ZnIn2S4, with X ) 0.0 wt% to 2.0 wt% X (wt%) 0.0 0.1 0.3 0.5 0.7 0.9 1.2 1.6 2.0 a

band gapa absorption PL emission emission intensity (eV) edge (nm) (nm) (normalized, a.u.) 2.51 2.16 2.11 2.05 2.02 1.94 1.91 1.73 1.65

495 575 587 604 614 639 650 715 750

560 597 611 617 626 632 639 655 666

0.33 0.72 0.76 1.00 0.74 0.56 0.42 0.20 0.19

Calculated from absorption edge.

be assigned to the transition from the Cu3d level above the valence band to the conduction band of ZnIn2S4. In addition, a broad band (λ > 750 nm) was observed for Cu (1.6 wt% to 2.0 wt%)-ZnIn2S4. This was due to the impurity CuS phase, as indicated by X-ray diffraction. The PL spectra of Cu(X)-ZnIn2S4, with X ) 0 wt% to 2.0 wt%, were obtained using an excitation wavelength of 330 nm, as shown in Figure 4. The PL intensity and response range of Cu(X)-ZnIn2S4 were influenced by Cu doping. The PL spectra, in company with the diffuse reflect spectra, shifted successively to longer wavelength with the change in the amount of Cu doped, indicating that the energy structure of Cu(X)-ZnIn2S4 depended on the value of X. Similar phenomena has been reported by Kudo et al. in the (AgIn)xZn2(1-x)S2 solid solutions.19 However, as shown in Table 1, the emission spectra of Cu(X)ZnIn2S4 (X ) 0.0 wt% to 0.7 wt%) was somewhat red-shifted vis-a-vis the absorption edge. This is because the PL emission of Cu(X)-ZnIn2S4 presented here is broadband,36 or donoracceptor defect (such as sulfur vacancy) based,37 in nature. In constrast, the absorption edge of Cu(X)-ZnIn2S4 (X ) 0.9 wt% to 2.0 wt%) shifted to longer wavelength when compared to the emission spectra. This may be due to the excess of Cu doping, resulting in the formation of CuS, which has a total absorption in the visible-light region. Thus, as reported by Peng et al. on Cu-ZnS,38 the PL emission of Cu(X)-ZnIn2S4 may arise from the recombination between the shallow donor level (sulfur vacancy) and the t2 level of Cu2+ (splitting from Cu3d). As the energy level of sulfur vacancy relative to the valence band nearly stays constant in these samples despite the variation in Cu2+ concentration, it can be concluded that the t2 energy level of

Cu2+ ions is farther from the valence band with increasing Cu2+ concentration. In addition, it could be found from Figure 4 that the PL intensity of Cu(X)-ZnIn2S4 gradually increased as the Cu content increased and arrived at the highest degree when the Cu content was 0.5 wt%. While the Cu content continued to increase, namely more than 0.5 wt%, the PL intensity began to decrease. In Mn-ZnS39 and Cu-ZnS,38 a similar photoluminescence phenomenon was also observed, which can be explained by the effect of ion doping. As the foregoing analysis shows, these PL spectra were related to native defects (e.g., sulfur vacancy). When Cu2+ was doped into ZnIn2S4, more defect states would be introduced. Therefore, it is reasonable that the defect-related PL intensities were enhanced for the Cu2+doped samples compared with the undoped sample. As for the decrease of the PL intensity with the Cu2+ concentration above 0.5 wt%, it may be caused by the formation of CuS, though the XRD measurement did not detect the existence of the copper sulfide phase. In Eu-doped GaN,40 a similar concentration quenching phenomenon was also observed, which was mainly attributed to the formation of EuN compound. 3.4. Band Structures. Figure 5A shows the band structure and the density of state (DOS) of ZnIn2S4. The density contour maps for the lowest unoccupied molecular orbitals (LUMO) and highest occupied molecular orbitals (HOMO) of ZnIn2S4 are shown in Figure 5B. We can observe clearly that both the top of valence band and the bottom of the conduct band lay at the G point of the Brillouin zone from Figure 5A. So the pure ZnIn2S4 crystal is a direct gap band semiconductor. The theoretical value of the direct gap at G is 0.4 eV, which is less than the experimental value of about 2.51 eV. Such an underestimation of the band gap is a well-known artifact of GGA. The density of states (Figure 5A) indicates that the S3p orbitals make a significant contribution to the valence band top of ZnIn2S4 and that the highest occupied molecular orbital (HOMO) levels are composed mainly of the hybridized S3p and Zn3d orbitals. The In5s orbitals locate at more negative energy levels than that of S3p orbitals and do not contribute much to the valence band top. The lowest unoccupied molecular orbital (LUMO) levels are composed mainly of the In5s5p and S3p orbitals. The Zn4s4p orbitals locate at more positive energy levels and do not contribute much to the conduction band bottom. Based on the optical properties and DFT calculations, the schematic energy level diagram of Cu(X)-ZnIn2S4 is shown in Figure 6. As for ZnIn2S4, upon photoexcitation, electrons would transfer from the valence band (hybridized S3p and Zn3d orbitals) to the conduction band (hybridized In5s5p and S3p orbitals), leaving photogenerated holes in the valence band. As aforementioned, the PL emission has been known to be due to the photoluminescence transition from the sulfur-vacancy-related donor level or the conduction band to the valence band. Therefore, the energy difference between the absorption edge and the emission spectra of ZnIn2S4 can be explained. Taking the Cu doping into account, the t2 state, splitting from discrete Cu3d levels, will form above the edge of valence band of ZnIn2S4, which is consistent with those reported by Peng38 and Xu41 in Cu-ZnS. A similar orbital-splitting phenomenon has been observed by Ye42 and Zou43 for Cr-3d and Ni-3d orbtials in In12NiCr2Ti10O42 and Ca2NiWO6. The t2(Cu3d) level, produced by doping Cu2+ into ZnIn2S4, works as the donor level and acceptor level for photoexcitation and PL emission, respectively,44 as shown in Figure 6. With the amount of Cu doped increasing, the t2(Cu3d) level will be elevated farther from the



Figure 5. (A) Band structure and density of states for ZnIn2S4 calculated by the density functional method. (B) Density contour maps for the LUMO and HOMO of ZnIn2S4.

Figure 6. Schematic energy level diagram of Cu(X)-ZnIn2S4. Vs stands for sulfur vacancy.

valence band, which results in the redshift of absorption edge and PL emission for Cu(X)-ZnIn2S4. 3.5. Photocatalytic Activities of Cu(X)-ZnIn2S4. Figure 7 shows the dependence of photocatalytic H2 evolution from an aqueous Na2SO3/Na2S solution over Cu(X)-ZnIn2S4 under visible-light irradiation (λ > 430 nm). Either nondoped or Cu-

Figure 7. Photocatalytic H2 evolution under visible-light irradiation over Cu(X)-ZnIn2S4; the values of X were (a) 0.0 wt%, (b) 0.1 wt%, (c) 0.3 wt%, (d) 0.5 wt%, (e) 0.7 wt%, (f) 0.9 wt%, (g) 1.2 wt%, (h) 1.6 wt%, (i) 2.0 wt%.


Shen et al. in Cu content not only raised the t2 (Cu3d) level to narrow the band gap of Cu(X)-ZnIn2S4, which has been demonstrated by the UV-vis diffuse reflectance spectra, but also enhanced the density of the t2 (Cu3d) levels and increased the concentration and the mobility of the photoexcited electrons.44 These could be related to the enhancement of photocatalytic activity. However, further increase in the amount of Cu doped would result in the increased absorption to longer wavelength, which was likely due to the formation of CuS, as discussed previously. Thus, the CuS impurity weakened the photoluminescence,40 as well as lowered photocatalytic activity of Cu-doped ZnIn2S4.11 4. Conclusions

Figure 8. Dependence of photocatalytic activity for H2 evolution and PL emission intensity over Cu(X)-ZnIn2S4, with X ) 0.0 wt% to 2.0 wt%.

doped ZnIn2S4 was active and stable for splitting water to hydrogen. The rate of H2 evolution over nondoped ZnIn2S4 was 26.1 µmol · h-1. The apparent quantum yield at 420 nm was calculated to be 9.6% by eq 1. As the amount of doped Cu was increased, the photocatalytic activity of Cu(X)-ZnIn2S4 was increased, because the visible-light absorption band of Cu(X)ZnIn2S4 grew.11 The highest activity was obtained when 0.5 wt % of Cu was doped, the rate of hydrogen evolution reached at 151.5 µmol · h-1, with the apparent quantum yield determined to be 14.2%. As the concentration of Cu doped was above 0.5 wt%, the activity was decreased, though the visible-light absorption band grew further. Such a similar dependence of photocatalytic H2 evolution upon the amount of dopant has been observed for several other photocatalysts.11,44,45 These observations indicated that the photocatalytic activites depended upon not only the visible-light absorption (i.e., band gap) but also some other factors. One of the reasons for the decrease in photocatalytic activity may be due to the impurity phase of CuS, resulted from the excess of Cu doping, as proved by XRD results. The CuS impurity may work as recombination sites between photogenerated electrons and holes. Another possible inactivation factor for the ZnIn2S4 doped with excessive Cu is that the layered structure could be destructed gradually with the amount of Cu increased further, as revealed by SEM images. Some perovskite photocatalysts with layered structure have also shown good photocatalytic activity for splitting water to hydrogen, as the dipole moment along layers seems to enhance the charge separation, resulting in high activity.46,47 Figure 8 shows the photocatalytic activity and photoluminescence intensity as a function of Cu content in the Cu(X)ZnIn2S4. The change in photoluminescence intensity was concurrent to that in photocatalytic activity, as both photoluminescence intensity and photocatalytic activity reached the highest level when 0.5 wt % of Cu was doped. Photoluminescence depends on various factors such as the densities of photoexcited charges and recombination centers, the extent of nonradiation process, and the mobility of photoexcited charges.44 It is difficult to determine which process is mainly responsible for changes in photoluminescence intensity with Cu content. However, in association with the change in photocatalytic activity, a plausible explanation can be made as follows. During the PL process, vacancies (such as sulfur vacancy) and defects can easily bind photoinduced electrons to form excitons, so that the PL signal can easily occur.48 When Cu2+ ions were doped into ZnIn2S4, more vacancies (such as sulfur vacancy) or defect states would be introduced.38 The larger the content of vacancy or defect, the stronger the PL signal. Meanwhile, an increase

In summary, we have synthesized a series of Cu-doped ZnIn2S4 photocatalysts by a simple hydrothermal method. The photocatalytic activity of ZnIn2S4 was remarkably enhanced by Cu doping. The optimized Cu (0.5 wt%)-doped ZnIn2S4 photocatalyst showed the highest activity for splitting water into H2, with the rate of hydrogen evolution to be 151.5 µmol · h-1. With increasing concentration of Cu doped, the UV-vis spectra and PL emission peak were systematically shifted to longer wavelength. Moreover, there were certain intrinsic relationships between the PL emission intensity and photocatalytic activity of Cu-doped ZnIn2S4. That is to say, the change in photoluminescence intensity was concurrent to that in photocatalytic activity, both depending on the increasing concentration of Cu doped. Acknowledgment. The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No.50521604) and National Basic Research Program of China (No.2003CB214500). The authors also would like to thank the molecular simulating platform and National High Performance Computing Center (Xi’an) at Xi’an Jiaotong University for the calculation resource support. References and Notes (1) Domen, K.; Kudo, A.; Ohnishi, T. J. Catal. 1986, 102, 92. (2) Hwang, D. W.; Kim, H. G.; Kim, J.; Cha, K. Y.; Kim, Y. G.; Lee, J. S. J. Catal. 2000, 193, 40. (3) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082. (4) Sato, J.; Saito, N.; Yamada, Y.; Maeda, K.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2005, 127, 4150. (5) Kim, H. G.; Hwang, D. W.; Lee, J. S. J. Am. Chem. Soc. 2004, 126, 8912. (6) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295. (7) Ritterskamp, P.; Kuklya, A.; Wu¨stkamp, M.; Kerpen, K.; Weidenthaler, C.; Demuth, M. Angew. Chem., Int. Ed. 2007, 46, 7770. (8) Niishiro, R.; Kato, H.; Kudo, A. Phys. Chem. Chem. Phys. 2005, 7, 2241. (9) Kudo, A.; Sekizawa, M. Catal. Lett. 1999, 58, 241. (10) Kudo, A.; Sekizawa, M. Chem. Commun. 2000, 15, 1371. (11) Tsuji, I.; Kudo, A. J. Photochem. Photobiol. A 2003, 156, 249. (12) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (13) Bubler, N.; Meier, K.; Reber, J. F. J. Phys. Chem. 1984, 88, 3261. (14) Naman, S. A.; Gra¨tzel, M. J. Photochem. Photobiol. A 1994, 77, 249. (15) Inoue, T.; Watanabe, T.; Fujishima, A.; Honda, K. J. Electrochem. Soc. 1977, 124, 719. (16) Shangguan, W.; Yoshida, A. J. Phys. Chem. B 2002, 106, 12227. (17) Shen, S.; Guo, L. J. Solid State Chem. 2006, 179, 2629. (18) Shen, S.; Guo, L. Mater. Res. Bull. 2008, 43, 437. (19) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Am. Chem. Soc. 2004, 126, 13406. (20) Tsuji, I.; Kato, H.; Kudo, A. Angew. Chem., Int. Ed. 2005, 44, 3565. (21) Jang, J. S.; Choi, S. H.; Shin, N.; Yub, C.; Lee, J. S. J. Solid State Chem. 2007, 180, 1110.

Enhanced Photocatalytic Hydrogen Evolution (22) Liu, G.; Zhao, L.; Ma, L.; Guo, L. Catal. Commun. 2008, 9, 126. (23) Romeo, N.; Dallaturca, A.; Braglia, R.; Sberveglieri, G. Appl. Phys. Lett. 1973, 22, 21. (24) Seo, W. S.; Otsuka, R.; Okuno, H.; Ohta, M.; Koumoto, K. J. Mater. Res. 1999, 14, 4176. (25) Romeo, N.; Tarricone, L.; Zanotti, L. Il NuoVo Cim. D 1983, 2, 2007. (26) Lei, Z.; You, W.; Liu, M.; Zhou, G.; Takata, T.; Hara, M.; Domen, K.; Li, C. Chem. Commun. 2003, 17, 2142. (27) Gou, X.; Cheng, F.; Shi, Y.; Zhang, L.; Peng, S.; Chen, J.; Shen, P. J. Am. Chem. Soc. 2006, 128, 7222. (28) Shen S.; Zhao L.; Guo L. Mater. Res. Bull., 2008, doi:10.1016/ j.materresbull.2008.03.027. (29) Shen S.; Zhao L.; Guo L. Int. J. Hydrogen Energy, 2008, doi:10.1016/j.ijhydene.2008.05.043. (30) Segall, M. D.; Lindan, P. J. D.; Probert, M. J.; Pickard, C. J.; Hasnip, P. J.; Clark, S. J.; Payne, M. C. J. Phys.: Condens. Matter 2002, 14, 2717. (31) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (32) Lei, Z.; Ma, G.; Liu, M.; You, W.; Yan, H.; Wu, G.; Takata, T.; Hara, M.; Domen, K.; Li, C. J. Catal. 2006, 237, 322. (33) Shionoya, S.; Tamoto, Y. J. Phys. Soc. Jpn. 1964, 19, 1142. (34) Konta, R.; Ishii, T.; Kato, H.; Kudo, A. J. Phys. Chem. B 2004, 108, 8992. (35) Miyauchi, M.; Takashio, M.; Tobimatsu, H. Langmuir 2004, 20, 232.

J. Phys. Chem. C, Vol. 112, No. 41, 2008 16155 (36) Spanhel, I.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826. (37) Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. J. Phys. Chem. B 2004, 108, 12429. (38) Peng, W. Q.; Cong, G. W.; Qu, S. C.; Wang, Z. G. Opt. Mater. 2006, 29, 313. (39) Ghosh, P. K.; Ahmed, Sk. F.; Jana, S.; Chattopadhyay, K. K. Opt. Mater. 2007, 29, 1584. (40) Bang, H.; Morishima, S.; Sawahata, J.; Seo, J.; Takiguchi, M.; Tsunemi, M.; Akimoto, K.; Nomura, M. Appl. Phys. Lett. 2004, 85, 227. (41) Xu, S. J.; Chua, S. J.; Liu, B.; Gan, L. M.; Chew, C. H.; Xu, G. Q. Appl. Phys. Lett. 1998, 73, 478. (42) Wang, D.; Zou, Z.; Ye, J. Chem. Phys. Lett. 2005, 411, 285. (43) Li, D.; Zheng, J.; Zou, Z. J. Phys. Chem. Solids 2006, 67, 801. (44) Arai, N.; Saito, N.; Nishiyama, H.; Domen, K.; Kobayashi, H.; Sato, K.; Inoue, Y. Catal. Today 2007, 129, 407. (45) Nishimoto, S.; Matsuda, M.; Miyake, M. Chem. Lett. 2006, 35, 308. (46) Kudo, A.; Kato, H.; Nakagawa, S. J. Phys. Chem. B 2000, 104, 571. (47) Hwang, D. W.; Kim, H. G.; Lee, J. S.; Kim, J.; Li, W.; Oh, S. H. J. Phys. Chem. B 2005, 109, 2093. (48) Jing, L.; Qu, Y.; Wang, B.; Li, S.; Jiang, B.; Yang, L.; Fu, W.; Fu, H.; Sun, J. Sol. Energy Mater. Sol. Cells 2006, 90, 1773.

JP804525Q

Enhanced Photocatalytic Hydrogen Evolution over Cu-Doped ZnIn2S4

Recommend Documents